Clarify custom bevy branch for now.

README.md

@@ -47,6 +47,7 @@ Because bevy's new renderer is not released yet there is no crate on crates.io y
 ```rust
 kayak_ui = { git="https://github.com/StarArawn/kayak_ui", rev="{INSERT_COMMIT_SHA_HERE}" }
 kayak_widgets = { git="https://github.com/StarArawn/kayak_ui", rev="{INSERT_COMMIT_SHA_HERE}" }
+bevy = { git="https://github.com/StarArawn/bevy", rev="{INSERT_COMMIT_SHA_HERE}" }
 ```
 
 It's also worth mentioning that you will need to use the same bevy revision that this library uses which is currently: `bcca341d696c66d0173d8b0ac7a1b23b4b9e775c`.